When taking the train from Glasgow, if you want seats around a table, make sure to reserve them in advance. If you're planning to tour the Oban whiskey distillery, make sure to book ahead, they can get sold out during the summer.
It rains in short spurts in the summer but it comes down fast and hard. We went in the middle of June and it was around 60℉ during the day, and with the wind it can feel cold.
The museum, castle and grounds are only open April through October.
If you are planning to explore Scotland by train I recommend getting the Highland Rover pass which allowed us 4 days of unlimited travel by train, ferry and bus over the course of 8 days.